test(plex): cover create write-through + the 409->nameTaken mapping (F-F)

Backend: create-collection happy path (write-through + editable) and the
create resync-failure 422 (honest reconcile detail), mirroring the rename/remove
tests. Frontend: extract notifyCollectionEditError (the 409->nameTaken vs
generic-fallback mapping that was inline in the New-collection create()) and
unit-test it; wire create() through it.

frontend/src/lib/actionErrors.test.ts

@@ -0,0 +1,31 @@
+// @vitest-environment jsdom
+//
+// jsdom (not node): importing ../api pulls in i18n, which reads navigator.language at module load.
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+// Spy on the toast sink so we can assert exactly what each error maps to.
+vi.mock("./notifications", () => ({ notify: vi.fn(() => 0) }));
+
+import { HttpError } from "./api";
+import { notify } from "./notifications";
+import { notifyCollectionEditError } from "./actionErrors";
+
+const notifyMock = vi.mocked(notify);
+beforeEach(() => notifyMock.mockClear());
+
+describe("notifyCollectionEditError", () => {
+  it("maps a backend 409 to the friendly nameTaken message (not the raw server detail)", () => {
+    notifyCollectionEditError(new HttpError(409, "already exists"), "EDIT_FAILED", "NAME_TAKEN");
+    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "NAME_TAKEN" });
+  });
+
+  it("falls back to the server detail for a non-409 HttpError", () => {
+    notifyCollectionEditError(new HttpError(500, "server boom"), "EDIT_FAILED", "NAME_TAKEN");
+    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "server boom" });
+  });
+
+  it("falls back to the generic edit-failed message for a non-HttpError", () => {
+    notifyCollectionEditError(new Error("network"), "EDIT_FAILED", "NAME_TAKEN");
+    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "EDIT_FAILED" });
+  });
+});